The present invention is in the technical field of manufacturing and checking value documents and relates to a value document having a security marking as well as to a method for the identification of the same.
Value documents are normally protected against an undesirable and often illegal duplication by a special label. It has been known for a long time to provide value documents for this purpose with luminescing substances having a specific emission behaviour.
The print WO 9916009 A1 describes the authenticity verification of a value document through a determination of the luminescence decay time of a security marking. Here, the security marking is excited in pulsed fashion and the time after the end of the excitation pulse, which passes until a predefined luminescence intensity is achieved, is determined.
Another method for the authenticity verification of a value document through a determination of the luminescence decay time of a security marking is disclosed in the print WO 0188846A1. In this method, the luminescence intensity is measured at several points in time after switching off an excitation pulse in order to determine the decay curve and to compare this with a target curve.
The print U.S. Pat. No. 7,762,468 B2 shows an authentication method in which a combination of two luminescent substances having different decay times is used. In this method, the second, more slowly decaying luminescent substance is captured only when the luminescence of the first luminescent substance has already decayed.
From the print DE 102006047851 A1 there can likewise be inferred the evaluation of a security marking having luminescent substances with different decay behaviours and overlapping emission spectra. In this method, the temporal course of the luminescence intensities is measured and for authentication the shape of the curve is compared to target values.
The print U.S. Pat. No. 9,046,486 B2 discloses a security marking and a method for identifying the same based on combinations of quasi-resonant luminescent substances having different exponential decay behaviours. With the help of a non-linear adaptation the amplitudes as well as the decay times are determined. The described method is not suitable for marking substances having a strongly non-exponential decay behaviour, which limits the number of available marking substances. Likewise, the analysis by means of non-linear adaptation turns out to be time-consuming and error-prone with respect to noise, so that the speed and quality of the evaluation is low.
With the luminescent substances and evaluating methods known in the prior art there can be achieved a satisfactory solution for the forgery-proof labelling of value documents, but, however, it is disadvantageous in particular in the case of luminescent substances having non-exponential decay behaviour that the combinatorial variety of the available luminescent substances is limited. This causes a restricted variability of the labelling, which may result, among other things, in a decrease of the forgery resistance. If luminescent substances having complex time behaviour are used as base substances for the coding of mixtures with different decay behaviours, the hitherto known evaluating methods, as they are known for example from the print U.S. Pat. No. 9,046,486B2, are unsuitable for securely assessing such security features in time-critical situations, as for example on high-speed bank note processing machines.